An ultrasound wave is normally a sound wave of 16,000 Hz or more, and is applied in various fields such as inspecting defects of an article, or diagnosing a disease, because the ultrasound wave can check the interior of an object non-destructively and non-invasively. One of the apparatuses utilizing an ultrasound wave is an ultrasound diagnostic apparatus, wherein a subject to be checked is scanned by an ultrasound wave, and an inner state of the subject is imaged based on a receiving signal generated from a reflection wave (an echo) of the ultrasound wave within the subject. The ultrasound diagnostic apparatus is provided with an ultrasound probe for transmitting/receiving an ultrasound wave with respect to a subject. The ultrasound probe generates an ultrasound wave by mechanical vibrations based on an electrical signal for transmission by utilizing a piezoelectric phenomenon. The ultrasound probe includes plural piezoelectric elements for generating an electrical signal for receiving by receiving a reflection wave of an ultrasound wave generated by mismatching of sound impedance within the subject, wherein the plural piezoelectric elements are arranged in e.g. two-dimensional arrays (see e.g. patent literature 1 (D1)).
In recent years, research and development have been made on the harmonic imaging technology of imaging an inner state of a subject, using a harmonic frequency component, in place of using a frequency (fundamental frequency) component of an ultrasound wave transmitted from an ultrasound probe to the interior of the subject. The harmonic imaging technology has various advantages: the contrast resolution is enhanced, because the side robe level is small as compared with the level of a fundamental frequency component, and the S/N ratio (signal to noise ratio) is increased; the resolution in a lateral direction is improved, because the beam width is reduced resulting from an increase in the frequency; multiple reflections are suppressed, because the sound pressure is small and a variation in sound pressure is small in a near-distance region; and a larger speed at a deep position can be secured, as compared with a case that a high frequency is used as a fundamental wave, because attenuation in a position farther from a focal point is substantially the same as that of the fundamental wave.
The ultrasound probe for use in the harmonic imaging technology requires a wide frequency band from a frequency of a fundamental wave to a frequency of a harmonic, a frequency range corresponding to a low frequency is utilized to transmit a fundamental wave, and a frequency range corresponding to a high frequency is utilized to receive a harmonic. An example of the ultrasound probe for use in the harmonic imaging technology is disclosed in patent literature 2 (D2).
FIG. 10 is a constructional diagram of a piezoelectric portion of the ultrasound probe disclosed in patent literature 2. FIG. 11 is an explanatory diagram of a method for manufacturing the piezoelectric portion of the ultrasound probe disclosed in patent literature 2.
Referring to FIG. 10, an ultrasound probe 500 disclosed in patent literature 2 includes a sound absorbing layer 501, a first piezoelectric layer 502 disposed on a front surface of the sound absorbing layer 501, a second piezoelectric layer 503 disposed on a front surface of the first piezoelectric layer 502, and a sound matching layer 504 disposed on a front surface of the second piezoelectric layer 503. The first piezoelectric layer 502 is constituted of first piezoelectric elements 5021 arranged in a certain direction. The first piezoelectric layer 502 has a thickness of one-half of a wavelength λ1 to be calculated based on a sound velocity inherent to the first piezoelectric layer 502, corresponding to a fundamental frequency f1. The second piezoelectric layer 503 is constituted of second piezoelectric elements 5031 arranged with the same pitch as the pitch of the first piezoelectric elements 5021 of the first piezoelectric layer 502. The second piezoelectric layer 503 has a thickness of one-fourth of a wavelength λ2 to be calculated based on a sound velocity inherent to the second piezoelectric layer 503, corresponding to a frequency f2, to receive an ultrasound wave of the frequency f2 of two times of the fundamental frequency f1. First electrodes 5051 used in common between the first piezoelectric elements 5021 and the second piezoelectric elements 5031 are formed between the first piezoelectric layer 502 and the second piezoelectric layer 503, with the same pitch as the pitch of the first piezoelectric elements 5021 and the second piezoelectric elements 5031 and by the same number as the number of the first piezoelectric elements 5021 and the second piezoelectric elements 5031. A second electrode 506 used in common between the first piezoelectric elements 5021 is formed between the first piezoelectric layer 502 and the sound absorbing layer 501. A third electrode 507 used in common between the second piezoelectric elements 5031 is formed between the second piezoelectric layer 503 and the sound matching layer 504. The ultrasound probe 500 disclosed in patent literature 2 is firmly contacted with a subject LB, whereby the ultrasound probe 500 is allowed to transmit/receive an ultrasound wave in a wide frequency band.
The ultrasound probe 500 disclosed in patent literature 2 is manufactured by the following steps. Referring to FIGS. 10 and 11, a first piezoelectric ceramic plate 5020 serving as the first piezoelectric layer 502 of a final product, and a second piezoelectric ceramic plate 5030 serving as the second piezoelectric layer 503 of the final product are placed one over the other, with a conductive mesh sheet coated with an electrode forming material, which serves as the first electrodes 5051 of the final product, being interposed therebetween, followed by baking. A second electrode 506 is formed in advance on the back surface of the first piezoelectric ceramic plate 5020. Subsequently, the two piezoelectric ceramic plates 5020 and 5030 placed one over the other are fixedly attached to the sound absorbing layer 501, and slits 5011 are formed. Thus, the first piezoelectric ceramic plate 5020 is formed into arrays of the first piezoelectric elements 5021, and the second piezoelectric ceramic plate 5030 is formed into arrays of the second piezoelectric elements 5031. The first electrodes 5051 arranged in a certain direction are also formed. Then, slits 5012 are formed in the second piezoelectric ceramic plate 5030 to such a depth that the first electrodes 5051 are not separated from each other. Then, a resin is impregnated into the slits 5011 and the slits 5012. After the resin is cured, the front surface of the second piezoelectric ceramic plate 5030 is abraded into a flat surface, and a third electrode 507 is formed by e.g. plating or vapor deposition. Then, the sound matching layer 504 is formed on the third electrode 507.
In the ultrasound probe having the above arrangement, as well as an ultrasound probe for use in harmonic imaging and laminated with first and second piezoelectric elements, it is necessary to provide a step of forming grooves (spacings, clearances, gaps, slits) in a piezoelectric plate in order to form plural piezoelectric elements out of the piezoelectric plate, divide the piezoelectric elements into groups depending on their functions, and individually operate the piezoelectric elements. Thus, a certain production cost for the ultrasound probe has been required.
Patent literature 1: JP 2004-088056A
Patent literature 2: JP Hei 11-276478A